Waveguide termination with magnetic metal walls wherein the curie temperature thereof is exceeded during operation



Sept. 7, 1965 R. ADAMS 3,

WAVEGUIDE TERMINATION WITH MAGNETIC METAL WALLS WHEREIN THE CURIETEMPERATURE THEREOF IS EXCEEDED DURING OPERATION Filed April 16, 1963//1/l/EA/70R ROBERT ADAMS GavmmmgMu-M United States Patent 3,205,459WAVEGUIDE TERMINATION WITH MAGNET- 'IC METAL WALLS WIT-HEREIN THE CURETEMPERATURE THEREOF IS EXCEEDED DURING OPERATION Robert Adams, Barnton,Edinburgh, Scotland, assignor to Ferranti, Limited, 'Hollinwood,Lancashire, England, a company of Great Britain and Northern IrelandFiled Apr. 16, "1963, Ser. No. 273,416 Claims priority, applicationGreat Britain, Apr. 25, 1962, 15,703/62 4 Claims. (Cl. 333-22) Thisinvention relates to waveguide systems and specifically to dissipativeloads for them.

Dissipative loads are available in a variety of forms and may berequired to absorb the total output power of a radio or radartransmitter when it is undesirable to radiate the power from an aerial.These conditions arise, for example, when setting up and adjusting atransmitter to its proper frequency and maximum power output.

Most of the available forms of dissipative load, however, havedisadvantages when used in this way. For example, the so-called waterload, in which a flow of water is maintained through a dielectric tubelocated within a section of waveguide, can be used to absorb high-powerenergy, but its use depends on a plentiful water supply or requires aclosed water-flow circuit including a heat exchanger. Moreover, thearrangement is fragile and the escape of water into the waveguide systemcould be disastrous.

Loads in which the power is absorbed in lossy dielectric wedges locatedinside the waveguide in contact with the walls are limited inpower-handling capacity because of poor heat transfer from the ceramicdielectric to the metal. These arrangements are also fragile.

A lossy ceramic material has been used to form a thickwalled Waveguidewith an internal taper for impedance matching purposes, energy beingabsorbed by the ceramic mass and radiated as heat. These also aremechanically weak both as regards the material itself, which is fran- Agible, and as regards the junction between the material and the metalflange which secures it to the waveguide system. This junction is liableto be broken by stresses due to differential expansion. Such material isporous and water absorbed by it may be released as steam to damage thestructure when the system is in operation. The material may be shieldedfrom the atmosphere by a metal cover but this cover must be in goodthermal contact with the material and the risk of breakage throughdifferential expansion is increased.

In order to dissipate high power radio frequency with such devices it isnecessary to use a number of load structures and a power-dividingnetwork in order to share the power amongst the loads. The cost andcomplexity of the installation is therefore increased.

An object of the invention is accordingly to provide for a waveguidesystem a high-power dissipative load which is to a large extent freefrom the disadvantages above set forth.

In accordance with the present invention, a high-power dissipative loadfor a waveguide system includes a length of dissipative waveguideshort-circuited at one end and made of magnetic metal material which hasan electrical resistance such that energy introduced at the other endthereof during operation is sufficiently attenuated by the time it isreceived back at said other end after reflection at the short-circuitedend.

The accompanying drawing is a view in perspective of one embodiment ofthe invention.

In carrying out the invention in accordance with one form by way ofexample, a dissipative load for a waveguide system includes a length ofrectangular waveguide having the sectional dimensions appropriate to thesystem. In order to occupy a convenient space the guide is coiled toform two flat helical coils 11 and 12, see the accompanying drawing,secured side by side on a frame, omitted from the drawing for clarity,which includes spacers between adjacent turns of each coil and, ifnecessary, is fitted with radiating fins to provide the necessary degreeof heat dissipation.

The inner ends of the coils are coupled together by a short portion ofwaveguide 13. The outer ends of the respective coils thus form the endsof the dissipative guide as a whole. One of these outer ends-that ofcoil 12, sayis short-circuited as indicated at 14 to reflect withoutappreciable loss at the point of reflection the energy reaching this endof the guide. The other end 15 is flanged for coupling to the waveguidesystem.

The material of the guide is a magnetic nickel-iron alloy having anappreciable electrical resistance and a Curie temperature the values ofwhich will be indicated shortly.

In operation, the energy entering the guide from the system at end 15gives rise to currents which, being at a high frequency, are confined bythe skin effect to flow mainly in the interior surface regions of theguide walls. The skin depth and hence the cross section available forcurrent flow is inversely dependent on the magnetic permeability of theguide material-that is, the skin depth is smaller with a magneticmaterial than with a nonmagnetic. Hence the use for the guide walls ofmagnetic material of appreciable permeability instead of the usualnon-magnetic material of unity permeability, together with theappreciable electrical resistance of the material, and possibly othermagnetic loss mechanisms such as hysteresis, imparts a heavy resistiveloss to the energy throughout its double journey along the guide to theshort circuit at 14 and back to the end 15. The resistance of thematerial is chosen such that, at the frequency concerned, this overallresistive loss has effected the required degree of attenuation by thetime the energy is returned from the guide to the system.

The energy dissipated appears as heat in the waveguide wall and thetemperature reached at any point along the length of the guide dependson the power level at that point. Since the power level is beingattenuated progressively along the guide a temperature gradient isproduced. As the temperature coefficient of resistance of most suitablewaveguide materials is positive, the resistivity of the hotter parts ofthe guide becomes higher than the resistivity of the cooler parts. Thetemperature gradient is therefore increased, since proportionately morepower is now being dissipated in the hot parts near the entrance to thedissipative load.

In a load of conventional construction this effect would be overcome,and a more uniform temperature distribution and power adsorptionachieved, by grading the resistivity or the dimensions of the lossymaterial. A somewhat similar eifect may be achieved in a loadconstructed according to the invention by making use of the change inthe magnetic properties of the material at the Curie temperature.

The material is chosen to have a Curie temperature which is reachedthrough the resistive heating of the guide walls in dissipating energyabove a predetermined power level. This power level is passed near theinput end of the waveguide load but not over the whole length. Theeffect of reaching this temperature is to reduce the permeability of theguide material substantially to unity, thereby removing the source ofmagnetic losses; thus the attenuation over this part of the waveguideload is lowered and more power is transmitted to more distant coolerparts of the guide. The temperature in these more distant parts isincreased, but beyond'a certain distance the power level is soattenuated that the Curie temperature is not reached. In this Way a moreuniform temperature distribution is achieved and in particular thetemperature near the input end of the waveguide load is prevented. frombecoming so high as to encourage arcing of the radio frequency poweracross the guide due to ionised particles provided by burnt dust, etc. I

In a particular construction the nickel-iron alloy of the guide materialhas an electrical conductivity of approximately 16,000 mhos percentimetre cube and a magnetic'permeability of approximately 2 at roomtemperature and at the working frequency of 10,000 mc./s., the Curiepoint being 360 C. A dissipative guide of that material suitable forinput powers of 2 or 3 kw. at the above frequency has a length of about27 feet. The cold loss is'about 0.5 db per foot and the hot loss 0.3 dbper foot for parts of the guide where the temperature is above the Curielevel.

It is usually desirable to design the supporting frame to allow for thedifierential expansion of the coiled guide caused by the largetemperature gradients along it when in operation, otherwiseparts of theguide may be overstressed,,in particular at the points where it isattached to the frame.

In addition to possessing the advantages of being of robust constructionand inexpensive to manufacture, a dissipative load in accordance withthe invention is free from the disadvantage of the known kind in whichthe energy is absorbed in interior dielectric wedges and as aconsequence there is poor heat transfer to the metal.

What I claim is:

1. A high-power dissipative load for a waveguide system including alength of dissipative waveguide short-' circuited at one end and havingwalls made of continuous magnetic metal material having a Curietemperature considerably above the ambient temperature, whereby parts ofsaidwalls are subject to a change in magnetic properties throughresistive heating thereof to a teml perature above the Curie temperatureduring normal operation.

2. Apparatus as claimed in claim 1 wherein the material is a magneticnickel-iron alloy.

3. Apparatus as claimed in claim 1 wherein the dissipative waveguide iscoiled to form two .fiat helical coils secured side by side and coupledtogether at their inner ends by a portion of waveguide extendingparallel to the common axis of the coils, the said one end which isshortcircuited being one of the outer ends.

4. A high-power dissipative load for a waveguide system including alength of dissipative waveguide shortcircuited at one end and havingwalls of uniform cross section made entirely of a magnetic nickel-ironalloy which has an electrical resistance such that energy introduced atthe other end thereof during operation is sufliciently attenuated by thetime it is received back at said other end after reflection at theshort-circuited end, said electrical resistance being such that theresistive heating of said walls during normal operation is eifective' toraise the temperature of a part of the dissipative Waveguide abovethe'Curie temperature of said nickel-iron alloy, whereby said rise intemperature serves to change the magnetic properties of said part of theWaveguide.

References Cited by the Examiner King: 'Measurements at CentimeterWavelength, Copyright 1952, Van Nostrand Co., Inc., New York, (page 57relied on).

Montgomery: Technique of Microwave Measurements, McGraw-Hill Book Co.,Inc., New York, Copyright 1947, pages 736 to 743 relied on.

Ragan: Microwave Transmission Circuits," Copyright 1948, McGraw-Hill,New York, (pages 115, 1 20 relied on).

Southworth: Principles and Applications of Waveguide Transmission,Copyright 1950, Van Nostrand, New York, pages 185, 244, 380 relied on).

HERMAN KARL SAALBACH, Primary Examiner.

1. A HIGH-POWER DISSIPATIVE LOAD FOR A WAVEGUIDE SYSTEM INCLUDING ALENGTH OF DISSIPATIVE WAVEGUIDE SHORTCIRCUITED AT ONE END AND HAVINGWALLS MADE OF CONTINUOUS MAGNETIC METAL MATERIAL HAVING A CURIETEMPERATURE CONSIDERABLY ABOVE THE AMBIENT TEMPERATURE, WHEREBY PARTS OFSAID WALLS ARE SUBJECT TO A CHANGE IN MAGNETIC PROPERTIES THROUGHRESISTIVE HEATING THEREOF TO A TEMPERTURE ABOVE THE CURIE TEMPERATUREDURING NORMAL OPERATION.